This invention relates to a process and apparatus for continuously monitoring biological reactions in domestic or industrial wastewater treatment plants. The successful operation of wastewater treatment facilities depends on close operational control over the entire process. Key to the required high degree of control lies in analysis of every step of the operation and every piece of equipment in the treatment plant, complementing the in-depth process technology to assure proper balance of chemical/mechanical interactions and/or their changes resulting from variations in influent quality or weather conditions. Such a scientific approach to everyday procedures, frequently ignored by municipalities due to lack of trained operators, permits valid comparisons with similar processes operated at other locations.
Process analysis involves breaking down the treatment operation into its component steps. Data bases of each step, taking into account the influent composition and flow rate, the plant configuration and size, and the operating efficiency of each piece of equipment, can define optimum manpower productivity. Anomalous behavior can be recognized as soon as operating parameters move beyond acceptable guidelines. This immediate feedback can precipitate fast corrective action to prevent unacceptable discharges which exceed compliance limits due to plant upsets or other causes. By automating mechanical operations, the possibility of human error in the process can be reduced. But more importantly, continuous data can provide a greater degree of sensitivity permitting control of processes and equipment. Having the ability to react quickly and precisely to minute process variations can afford a delicate, controlled balance to the overall operation.
This invention involves process analysis of the treatment of domestic and industrial wastewater. To promote aerobic biological processes, oxygen is added by aeration, trickling filters, rotating biological discs, pure oxygen gas, or other oxygen dispensing systems. The microbial use of available oxygen causes changes in the level of alkalinity. "Alkalinity" is defined as the ability to buffer acids determined by titrating with sulfuric acid to a select end point of 4.5 pH. (Note: pH is defined as the negative logarithm of the effective hydrogen-ion concentration or activity in gram equivalents per liter. It is used in expressing both acid and base activity where a value of 7 represents neutrality, values less than 7 are increasingly acidic and values greater than 7 increasingly alkaline.)
Alkalinity in sewage (wastewater) is due to the presence of hydroxides, carbonates, and bicarbonates of elements such as calcium, magnesium, sodium, potassium, or of ammonia and amines. Of these, bicarbonates are most common. Sewage is normally alkaline, receiving its alkalinity from the water supply, the ground water, and the materials added during domestic usage.
Alkalinity also may be viewed as the measure of the capacity of water to absorb hydrogen ions without significant pH change (i.e. to neutralize acids).
The hydrogen ion activity (i.e. intensity of the acid or alkaline condition of a solution) is referenced by the term pH, which is defined as: pH=log 1/(H.sup.+). Water dissociates to only a slight degree, yielding hydrogen ions equal to 10.sup.-7 mole/L, thus pure water has a pH of 7. It is also neutral since 10.sup.-7 mole/L of hydroxide ion is produced simultaneously (H.sub.2 O.fwdarw.H.sup.+ +OH.sup.-)
When an acid is added to water the hydrogen ion concentration increases, resulting in a lower pH value. Addition of an ,alkali can reduce the number of free hydrogen ions, causing an increase in pH because OH.sup.- ions unite with H.sup.+ ions. The pH scale is acidic below 7 and basic above 7. Below pH 4.5, dissolved CO.sub.2 predominates over bicarbonates (HCO.sub.3.sup.-) in solution, thus by our definition, no alkalinity exists. Between pH 4.5 and 8.3, bicarbonate ions (HCO.sub.3.sup.-) predominate. Above 8.3, the bicarbonates are dominated by carbonate ions (CO.sub.3.sup.2-). Hydroxide appears at a pH greater than 9.5 and reacts with CO.sub.2 to yield both bicarbonates and carbonates. ##STR1##
The alkalinity of water may be due to one or more of the following ionic forms; OH.sup.- (hydroxide), CO.sub.3.sup.2- (carbonates) and HCO.sub.3.sup.- (bicarbonate). For commonality the concentrations of these various radicals are given as mg of CaCO.sub.3 /L.
In wastewater treatment, pH must be maintained in a range favorable for biological activity. Observation and testing has demonstrated that this pH range is 6.5 to 8.0, with optimum activity between 6.7 and 7.3. Within this pH range a change of 0.3 pH units may result in a change of the alkalinity level by 200 mg/L or more. Because of the greater magnitude of alkalinity change, and its linearity as compared to pH, the measurement of alkalinity is more reliably monitored than pH and changes are more easily recognized, thus permitting quicker reaction on the part of the operators to adjust biological process activity to normal levels.
It has been clearly demonstrated that while operators may not react to a small change in pH, they will react to a broad alkalinity change. A broad swing of alkalinity level can occur during a period when the change in pH is only 0.1 to 0.3.
When titrating to an end point of 4.5, there is reason to believe that below a pH of 5.5, much of the alkalinity will result from insoluble bases as they go into solution. There is some doubt that insoluble bases are involved in the biological process, therefore, a lower pH end point below 5.75 may not necessarily need to be considered in establishing this alkalinity relationship. Similar results can be obtained by filtering the samples prior to analysis.
The major source of alkalinity in most wastewater treatment processes is the level of CO.sub.2 and NH.sub.3 (basic ammonia) present in the system. Excess aeration results in an increase in acid production (as CO.sub.2) and in increased nitrification which reduces the level of ammonia. Both acid production and ammonia removal cause a drop in alkalinity. A significant drop in alkalinity can be remedied by reducing the oxygen supply and/or increasing the solids level (i.e. concentration and feed of microbes). An increase in alkalinity is usually associated with an increase in ammonia. The alkalinity can be reduced by increasing the oxygen supply and/or reducing the solids level. Adjusting the oxygen supply in response to a change in alkalinity is the easiest, quickest and most logical method of bringing the biological activity back into balance. Adjusting the solids concentration (e.g. mixed liquor suspended solids (hereafter "MLSS"), or return activated sludge (hereafter "RAS") requires adjusting the volume and detention times, thus it is the secondary response to an alkalinity change.
Flows into wastewater plants vary daily and hourly but on a consistent basis. Normally, at about 0600 hours each day, flow begins to increase and at about 2200 hours, begins to drop. There may be smaller regular deviations in a particular collection system. Using the alkalinity level of the raw sewage as a base line, a 24-hour alkalinity profile of the treatment process can be quickly established.
There is a dramatic change in alkalinity immediately after discharge or mixing of raw sewage into the first biological process tank, such as aerated grit removal tanks, flow equalization tanks, primary settling tanks, aeration tanks or flow mixing tanks from side stream return. This change in alkalinity is caused by dilution so process alkalinity monitoring of base line may begin at start of first dilution area or biological process zone.
Once a 24 hour flow profile has been established, a change in alkalinity will indicate the need to increase or decrease the oxygen supply or solids inventory. This may be programmed into the system or be dependent upon operator functioning. Using this monitoring process can result in energy savings and a consistent improvement in effluent quality.
The alkalinity is continually changing in each of the aerobic or anaerobic process areas, including primary clarifier, aeration basin, secondary clarifier, sludge return, and digester. The rate of the biological processes occurring within the treatment system is dependent on a number of factors, including the supply of dissolved oxygen, the solids concentration, the degree of mixing and the temperature. Changes in biological activity produce changes in alkalinity levels, while the change in other parameters, such as pH, may be negligible.
The changes in alkalinity levels can occur rapidly and over a relatively wide range, indicating to the operator that operational changes need to be made before the normal profile of the process system is upset. The operator may make the necessary operational changes by manually adjusting the oxygen supply and/or pump speeds. Through the use of a programmed process control system, integrating pumps and air supplies with the monitor information, the adjustments can be made automatically to provide continuous control and real time reactions to the situation.
An increase in alkalinity indicates a shift in the biological activity which can be remedied by increasing the dissolved oxygen, decreasing the solids concentration, or both. A decrease in alkalinity indicates excess oxygen, nitrification, or an insufficient biological mass.
According to one aspect of the present invention there is provided a method of controlling an aerobic wastewater process, having an influent of wastewater to the process and an effluent of treated water from the process. The method comprises the following steps: (a) Determining a baseline of alkalinity by measuring the alkalinity profile of the influent wastewater to the aerobic wastewater treatment process. (b) Sensing the alkalinity of the wastewater being treated at a plurality of different points in the aerobic wastewater treatment process. (c) When sensed alkalinity from step (b) is above a predetermined amount over baseline alkalinity and on the basis of the alkalinity sensing alone, independent of dissolved oxygen sensing, increasing air supply and/or reducing concentration of microbes in foods until the sensed alkalinity level is within the predetermined amount over baseline; and (d) when the sensed alkalinity from step (b) is below a predetermined amount under baseline alkalinity, and on the basis of the alkalinity sensing alone, independent of dissolved oxygen sensing, decreasing air supply and/or increasing the concentration of food and microbes, until the sensed alkalinity level is within a predetermined amount under baseline. Efficient treatment of wastewater to produce treated effluent without the addition of outside alkalinity adjusting chemicals or additives is thus accomplished.
In the aerobic treatment of wastewater in which oxygen is supplied to microbes and feed, the invention comprises a method of maintaining alkalinity within a predetermined range so as to maintain the pH of the wastewater between about 6.5-8.0 (preferably 6.7-7.3) during treatment. The method steps are: (a) sensing the alkalinity of the wastewater at a plurality of points in the aerobic treatment of the wastewater; and (b) adjusting the alkalinity adjacent these points so that it is within about 100 mg CaCO.sub.3 /L of a desired value by changing the rate of oxygen supply and/or the concentration of microbes and feed, so as to control alkalinity to maintain pH within the desired range without the addition of chemicals or additives.
The invention also contemplates an apparatus for controlling an aerobic wastewater treatment process, which process includes a clarifier, aerobic treatment zone, and digester. The apparatus comprises: A plurality of alkalinity sensors, at least one associated with each of the clarifier, aerobic treatment zone and digester. Means for supplying oxygen to the aerobic treatment zone and digester, and means for withdrawing sludge from the clarifier. And, control means (i.e., a computer interface) for controlling the oxygen supply and sludge withdrawal in response to the alkalinity sensors.
The benefits to be derived from the method and apparatus of continuously monitoring and reacting to changes in the alkalinity level according to the invention have been clearly demonstrated. The data collected in test programs has provided convincing evidence that monitoring the change in the levels of alkalinity can be a very reliable indicator of change in microbial activity, thereby becoming a valuable tool for operators to utilize in making early process control changes to prevent process upset, instead of responding to an upset situation once it has occurred.
The increased process efficiency, with relatively little operator involvement, also improves sludge thickening by enhanced supernating in the digester. The end result is reductions in operating costs and odor control problems, while insuring a stabilized effluent quality. Further, automatic programming can reduce a plant's manpower requirements.
It is the primary object of the present invention to provide efficient control of an aerobic wastewater treatment process by sensing and maintaining an alkalinity level. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.